A. Field of Invention
This invention pertains to implantable devices such as cochlear electrodes incorporating bioresorbable materials, and more particularly to a device which has a first preselected shape suitable for insertion into the body of a patient, and a second shape suitable for providing a specific function, or stimulus, the bioresorbable materials being used to change the device from the second to the first shape.
B. Background of the Invention
The invention is described for electrodes used in cochlear implant systems, however, it is equally applicable to other implantable devices. Cochlear implant systems are used to aid patients having a hearing deficiency. More particularly, these systems include a microphone receiving ambient sounds and converting the sounds into corresponding electrical signals, signal processing means of processing the electrical signals and generating cochlea stimulating signals and an electrode assembly for applying the cochlea stimulating signals to the cochlea of the patient. It is known in the art that the cochlea is tonotopically mapped. In other words, the cochlea can be partitioned into regions, with each region being responsive to signals in a particular frequency range. This property of the cochlea is exploited by providing the electrode assembly with an array of electrodes, each electrode being arranged and constructed to deliver a cochlea stimulating signal within a preselected frequency range to the appropriate cochlea region. The electrical currents and electric fields from each electrode stimulate the cilia disposed on the modiola of the cochlea. Several electrodes may be active simultaneously.
It has been found that in order for these electrodes to be effective, the magnitude of the currents flowing from these electrodes and the intensity of the corresponding electric fields, are a function of the distance between the electrodes and the modiola. If this distance is great, the threshold current magnitude must be larger than if the distance is smaller. Moreover, the current from each electrode may flow in all directions, and the electrical fields corresponding to adjacent electrodes may overlap thereby causing cross-electrode interference. In order to reduce the threshold stimulation amplitude and to eliminate cross electrode interference, it is advisable to keep the distance between the electrode array and the modiola as small as possible. This is best accomplished by providing the electrode array in the shape which generally follows the shape of the modiola. Of course during insertion, the electrode assembly should be generally straight, because otherwise the insertion procedure is too cumbersome and difficult.
Several methods and means of curving the electrode assembly, however in the opinion of the inventors, none of these prior methods are satisfactory. For example, one electrode assembly is known which includes an electrode carrier provided with a longitudinal element arranged on one side of the carrier and constructed to change its dimension once the assembly is inserted. For example, the longitudinal element could include a hydrogel such as PAA (Polyacrylic acid) which expands after insertion by absorbing water from the cochlear fluid. Alternatively, the longitudinal element could be a bimetallic filament (such as nickel/titanium) which is shaped to allow the electrode carrier to take a straight configuration at room temperature but bends into a preselected shape once it is exposed to body temperature.
Another proposed electrode assembly included a mechanical member arranged to bend the electrode carrier after the carrier has been inserted.
All these prior art devices require a structure which is difficult and expensive to manufacture and which in most cases are not expected to perform satisfactorily.